The horizontal directional drilling (“HDD”) industry traditionally uses walk-over tracking techniques to follow the progress of a bore, to find the surface location immediately above the drill bit, and to determine the depth of the drill bit from that surface location. The primary tracking tools are a subsurface transmitter and a hand-carried surface receiver. The transmitter, located in or very near a boring tool, generally emits a magnetic dipole field created by a single coil dipole antenna. The transmitted dipole field can be used for both location and communication with the above ground receiver.
Conventional receivers often contain an arrangement of three antennas mounted in each of the three Cartesian axes. When the antenna arrangement senses the dipole field, the output of each antenna is proportional to the magnitude of the magnetic flux density as detected along the axis of the particular antenna. The signals from the antennas are mathematically resolved to provide information about the relative location of the boring tool. The process of locating the dipole, and thus the boring tool, currently involves two steps: determining its location along the z-axis (fore and aft) and then along the y-axis (left and right). One skilled in the art will appreciate a receiver can locate a transmitter in the fore-aft direction (along the z-axis) using the amplitude and phase of the transmitter's generated horizontal and vertical field components as measured in the vertical plane normal to the surface and extending through the transmitter axis (the x-z plane). A receiver can also determine the location of a single transmitter in the left-right directions using the amplitude and phase of the dipole field in the horizontal plane (the y-z plane). However, the left-right determination can only be used either in front of or behind the transmitter because there is no y component to the dipole field when the receiver is directly above the transmitter (such that z=0). There remains a need for improved tracking systems for simultaneously locating the transmitter in both the fore-aft and left-right directions with an antenna arrangement positioned directly over the transmitter.